Recently, there have frequently been used high-strength steel sheets having a tensile strength of not less than 50 kg/mm.sup.2 in high-strength members such as bumper, door guard bar and the like from the standpoints of the safety and the weight-saving of automotive vehicles. The materials for use in these applications are required to have the properties that the tensile strength is high, and the ductility is good, while the spot weldability as well as the resistance to secondary cold-work embrittlement are good at or after the assembling of the vehicle body. Lately, there have been used steel sheets having a dual-phase structure composed of ferrite and a low temperature transformation product consisting mainly of martensite as a steel sheet satisfying the above requirements. However, in order to increase the strength in such conventional dual-phase structure steel sheets, it is necessary to add alloying elements such as Mn, Si, Nb, Ti and the like in large quantities and consequently the production cost increases. Further, the addition of a large amount of Mn, Si or the like is apt to cause surface oxidation during the continuous annealing, resulting in the deterioration of the spot weldability and the resistance to secondary cold-work embrittlement. Therefore, it is difficult to cheaply manufacture a high-strength steel sheet having excellent ductility, spot weldability and resistance to secondary cold-work embrittlement up to now.
It is an object of the invention to eliminate the above problems of the prior art, and to provide a method of manufacturing a low yield ratio, highstrength steel sheet which is cheap in the production cost and has good ductility, resistance to secondary cold-work embrittlement, spot weldability and the like.